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Q. How can I use the EXTRACT commands to
find the depletion widths in my MOSFET at different drain biases
?

A. The EXTRACT feature within the DeckBuild application
is a powerful feature but requires experience to write the correct
syntax. We shall show how it may be used on a structure file to
find depletion widths.

To illustrate the use of this statement to find
depletion widths we have used the standard MOSFET example mos1ex01.in
to first create the structure. Then the drain voltage was swept
to 5V and a structure was saved at this bias. Figure 1 shows this
structure and has plottted the junction and the depletion edges.

Figure 1. The standard MOSFET structure with
the
drain biased to 5v that shows the depletion regions.

To extract the depletion width from a particular
structure we must first define what is the edge of the depletion
region. Typically the depletion edge is assumed to be where the
majority carrier concentration is equal to one half of the doping
concentration. Once the location of the two edges are found the
depletion width can be calculated. We shall now describe how this
can be translated into EXTRACT syntax.

First EXTRACT is initialied with the appropriate
file which in this case is the device with 5V applied. Any following
EXTRACT statements will operate on this two-dimensional structure
file.

extract init inf=”vd5v.str”

Next we calculate from the surface of the silicon
the depth at which the calculated electron carrier concentration
is equal to one half of the donor doping concentration. We have
selected a point at x = 1.1µm to extract the depletion edge
“Dxn”

Next we calculate from the surface of the silicon
the depth at which the calculated hole carrier concentration is
equal to one half of the acceptor doping concentration. We have
selected a point at x = 1.1um to extract the depletion edge “Dxp”

Finally we can use the ability of EXTRACT to perform
calculations on EXTRACT variable names to find the depletion width.

extract name=”Depletion Width @ x=1.1um
= “ $Dxp - $Dxn

For the device in Figure 1 the results from
this extraction are shown in Figure 2. The depletion width of 0.29um
is the same as the depletion width shown in Figure 1. This analysis
allows an easy way for the depletion width to be extracted either
at different x coordinates or from different structures which have
different applied voltages.

Figure 2. DeckBuild window that shows the
runtime output of the EXTRACT commands.